Author:

Barry P. Rand(IMEC, Kapeldreef 75, B-3001 Leuven Belgium)

The optimization of organic solar cells involves a fundamental tradeoff
between optical absorption length, mobility, and exciton diffusion length
($L_{D})$. Organic semiconductors possess $L_{D}$ that are at least one order
of magnitude less than their respective absorption lengths, meaning that
many excitons decay before reaching a dissociating interface. The bulk
heterojunction concept, whereby one mixes donor and acceptor components into
a single layer, is an effective way to avoid this bottleneck. However,
because mixed layers tend to have lower mobilities compared with pure films,
carrier transport in devices composed of mixed layers thick enough to absorb
a significant amount of light is poor, producing an inefficient device.
In this talk, we explore two promising approaches to solve these challenges.
In one, we investigate the possibility of increasing $L_{D}$ of a given
material. By employing a properly chosen phosphorescent dopant, we are able
to sensitize a population of long-lived triplet excitons in a normally
fluorescent material, increasing the diffusion length by more than a factor
of 2.
In another approach, we look into the possibility of exploiting surface
plasmon resonances of metal nanoparticles. These surface plasmon resonances
lead to strongly enhanced near fields, increasing absorption of nearby
chromophores. With this approach, therefore, the thickness of organic
semiconductor layers containing metal nanoparticles could be reduced without
compromising absorption. Here, we investigate exciton-plasmon interactions
through photoluminescence and absorption measurements of thin-films
consisting of organic semiconductors and metal nanoparticles, as a function
of film thickness with and without the presence of spacer layers between the
nanoparticles and absorbers. From this knowledge, we assess the prospect of
using plasmonic effects in thin film organic solar cells.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2010.MAR.B16.8